.\" Copyright (C) 2011, Eric Biederman <ebiederm@xmission.com>
.\" and Copyright (C) 2011, 2012, Michael Kerrisk <mtk.manpages@gmail.com>
.\"
.\" %%%LICENSE_START(GPLv2_ONELINE)
.\" Licensed under the GPLv2
.\" %%%LICENSE_END
.\"
.TH SETNS 2 2016-03-15 "Linux" "Linux Programmer's Manual"
.SH NAME
setns \- reassociate thread with a namespace
.SH SYNOPSIS
.nf
.BR "#define _GNU_SOURCE" "             /* See feature_test_macros(7) */"
.B #include <sched.h>
.sp
.BI "int setns(int " fd ", int " nstype );
.fi
.SH DESCRIPTION
Given a file descriptor referring to a namespace,
reassociate the calling thread with that namespace.

The
.I fd
argument is a file descriptor referring to one of the namespace entries in a
.I /proc/[pid]/ns/
directory; see
.BR namespaces (7)
for further information on
.IR /proc/[pid]/ns/ .
The calling thread will be reassociated with the corresponding namespace,
subject to any constraints imposed by the
.I nstype
argument.

The
.I nstype
argument specifies which type of namespace
the calling thread may be reassociated with.
This argument can have one of the following values:
.TP
.BR 0
Allow any type of namespace to be joined.
.TP
.BR CLONE_NEWCGROUP " (since Linux 4.6)"
.I fd
must refer to a cgroup namespace.
.TP
.BR CLONE_NEWIPC " (since Linux 3.0)"
.I fd
must refer to an IPC namespace.
.TP
.BR CLONE_NEWNET " (since Linux 3.0)"
.I fd
must refer to a network namespace.
.TP
.BR CLONE_NEWNS " (since Linux 3.8)"
.I fd
must refer to a mount namespace.
.TP
.BR CLONE_NEWPID " (since Linux 3.8)"
.I fd
must refer to a descendant PID namespace.
.TP
.BR CLONE_NEWUSER " (since Linux 3.8)"
.I fd
must refer to a user namespace.
.TP
.BR CLONE_NEWUTS " (since Linux 3.0)"
.I fd
must refer to a UTS namespace.
.PP
Specifying
.I nstype
as 0 suffices if the caller knows (or does not care)
what type of namespace is referred to by
.IR fd .
Specifying a nonzero value for
.I nstype
is useful if the caller does not know what type of namespace is referred to by
.IR fd
and wants to ensure that the namespace is of a particular type.
(The caller might not know the type of the namespace referred to by
.IR fd
if the file descriptor was opened by another process and, for example,
passed to the caller via a UNIX domain socket.)

.B CLONE_NEWPID
behaves somewhat differently from the other
.I nstype
values:
reassociating the calling thread with a PID namespace changes only
the PID namespace that child processes of the caller will be created in;
it does not change the PID namespace of the caller itself.
Reassociating with a PID namespace is allowed only if the
PID namespace specified by
.IR fd
is a descendant (child, grandchild, etc.)
of the PID namespace of the caller.
For further details on PID namespaces, see
.BR pid_namespaces (7).

A process reassociating itself with a user namespace must have the
.B CAP_SYS_ADMIN
.\" See kernel/user_namespace.c:userns_install() [3.8 source]
capability in the target user namespace.
Upon successfully joining a user namespace,
a process is granted all capabilities in that namespace,
regardless of its user and group IDs.
A multithreaded process may not change user namespace with
.BR setns ().
It is not permitted to use
.BR setns ()
to reenter the caller's current user namespace.
This prevents a caller that has dropped capabilities from regaining
those capabilities via a call to
.BR setns ().
For security reasons,
.\" commit e66eded8309ebf679d3d3c1f5820d1f2ca332c71
.\" https://lwn.net/Articles/543273/
a process can't join a new user namespace if it is sharing
filesystem-related attributes
(the attributes whose sharing is controlled by the
.BR clone (2)
.B CLONE_FS
flag) with another process.
For further details on user namespaces, see
.BR user_namespaces (7).

A process may not be reassociated with a new mount namespace if it is
multithreaded.
.\" Above check is in fs/namespace.c:mntns_install() [3.8 source]
Changing the mount namespace requires that the caller possess both
.B CAP_SYS_CHROOT
and
.BR CAP_SYS_ADMIN
capabilities in its own user namespace and
.BR CAP_SYS_ADMIN
in the target mount namespace.
See
.BR user_namespaces (7)
for details on the interaction of user namespaces and mount namespaces.

Using
.BR setns ()
to change the caller's cgroup namespace does not change
the caller's cgroup memberships.
.SH RETURN VALUE
On success,
.BR setns ()
returns 0.
On failure, \-1 is returned and
.I errno
is set to indicate the error.
.SH ERRORS
.TP
.B EBADF
.I fd
is not a valid file descriptor.
.TP
.B EINVAL
.I fd
refers to a namespace whose type does not match that specified in
.IR nstype .
.TP
.B EINVAL
There is problem with reassociating
the thread with the specified namespace.
.TP
.\" See kernel/pid_namespace.c::pidns_install() [kernel 3.18 sources]
.B EINVAL
The caller tried to join an ancestor (parent, grandparent, and so on)
PID namespace.
.TP
.B EINVAL
The caller attempted to join the user namespace
in which it is already a member.
.TP
.B EINVAL
.\" commit e66eded8309ebf679d3d3c1f5820d1f2ca332c71
The caller shares filesystem
.RB ( CLONE_FS )
state (in particular, the root directory)
with other processes and tried to join a new user namespace.
.TP
.B EINVAL
.\" See kernel/user_namespace.c::userns_install() [kernel 3.15 sources]
The caller is multithreaded and tried to join a new user namespace.
.TP
.B ENOMEM
Cannot allocate sufficient memory to change the specified namespace.
.TP
.B EPERM
The calling thread did not have the required capability
for this operation.
.SH VERSIONS
The
.BR setns ()
system call first appeared in Linux in kernel 3.0;
library support was added to glibc in version 2.14.
.SH CONFORMING TO
The
.BR setns ()
system call is Linux-specific.
.SH NOTES
Not all of the attributes that can be shared when
a new thread is created using
.BR clone (2)
can be changed using
.BR setns ().
.SH EXAMPLE
The program below takes two or more arguments.
The first argument specifies the pathname of a namespace file in an existing
.I /proc/[pid]/ns/
directory.
The remaining arguments specify a command and its arguments.
The program opens the namespace file, joins that namespace using
.BR setns (),
and executes the specified command inside that namespace.

The following shell session demonstrates the use of this program
(compiled as a binary named
.IR ns_exec )
in conjunction with the
.BR CLONE_NEWUTS
example program in the
.BR clone (2)
man page (complied as a binary named
.IR newuts ).

We begin by executing the example program in
.BR clone (2)
in the background.
That program creates a child in a separate UTS namespace.
The child changes the hostname in its namespace,
and then both processes display the hostnames in their UTS namespaces,
so that we can see that they are different.

.nf
.in +4n
$ \fBsu\fP                   # Need privilege for namespace operations
Password:
# \fB./newuts bizarro &\fP
[1] 3549
clone() returned 3550
uts.nodename in child:  bizarro
uts.nodename in parent: antero
# \fBuname \-n\fP             # Verify hostname in the shell
antero
.in
.fi

We then run the program shown below,
using it to execute a shell.
Inside that shell, we verify that the hostname is the one
set by the child created by the first program:

.nf
.in +4n
# \fB./ns_exec /proc/3550/ns/uts /bin/bash\fP
# \fBuname \-n\fP             # Executed in shell started by ns_exec
bizarro
.in
.fi
.SS Program source
.nf
#define _GNU_SOURCE
#include <fcntl.h>
#include <sched.h>
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>

#define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); \\
                        } while (0)

int
main(int argc, char *argv[])
{
    int fd;

    if (argc < 3) {
        fprintf(stderr, "%s /proc/PID/ns/FILE cmd args...\\n", argv[0]);
        exit(EXIT_FAILURE);
    }

    fd = open(argv[1], O_RDONLY);  /* Get file descriptor for namespace */
    if (fd == \-1)
        errExit("open");

    if (setns(fd, 0) == \-1)        /* Join that namespace */
        errExit("setns");

    execvp(argv[2], &argv[2]);     /* Execute a command in namespace */
    errExit("execvp");
}
.fi
.SH SEE ALSO
.BR clone (2),
.BR fork (2),
.BR unshare (2),
.BR vfork (2),
.BR namespaces (7),
.BR unix (7)
